Large-scale suspended sediment transport and sediment deposition in the Mekong Delta

Abstract

Abstract. Sediment dynamics play a major role in the agricultural and fishery productivity of the Mekong Delta. However, the understanding of sediment dynamics in the delta, one of the most complex river deltas in the world, is very limited. This is a consequence of its large extent, the intricate system of rivers, channels and floodplains, and the scarcity of observations. This study quantifies, for the first time, the suspended sediment transport and sediment deposition in the whole Mekong Delta. To this end, a quasi-2D hydrodynamic model is combined with a cohesive sediment transport model. The combined model is calibrated using six objective functions to represent the different aspects of the hydraulic and sediment transport components. The model is calibrated for the extreme flood season in 2011 and shows good performance for 2 validation years with very different flood characteristics. It is shown how sediment transport and sediment deposition is differentiated from Kratie at the entrance of the delta on its way to the coast. The main factors influencing the spatial sediment dynamics are the river and channel system, dike rings, sluice gate operations, the magnitude of the floods, and tidal influences. The superposition of these factors leads to high spatial variability of sediment transport, in particular in the Vietnamese floodplains. Depending on the flood magnitude, annual sediment loads reaching the coast vary from 48 to 60 % of the sediment load at Kratie. Deposited sediment varies from 19 to 23 % of the annual load at Kratie in Cambodian floodplains, and from 1 to 6 % in the compartmented and diked floodplains in Vietnam. Annual deposited nutrients (N, P, K), which are associated with the sediment deposition, provide on average more than 50 % of mineral fertilizers typically applied for rice crops in non-flooded ring dike floodplains in Vietnam.Through the quantification of sediment and related nutrient input, the presented study provides a quantitative basis for estimating the benefits of annual Mekong floods for agriculture and fishery, and is an important piece of information with regard to the assessment of the impacts of deltaic subsidence and climate-change-related sea level rise on delta morphology.

Has dyke development in the Vietnamese Mekong Delta shifted flood hazard downstream?

“…Moderate floods, commonly perceived as "good floods" by the local population, bring various benefits, e.g. reduction in soil acidity; removal of residual pesticides and other pollutants in paddy fields; an immense wealth of wild fish in the rivers, channels and inundated floodplains; and provision of nutrients through deposited sediments on the floodplains (Hashimoto, 2001;Sakamoto et al, 2007;Hung et al, 2012;Manh et al, 2013Manh et al, , 2014.…”

“…The model domain includes the CFP, the Tonle Sap Lake and the majority of the channels and hydraulic structures in the VMD. The model was initially developed by Dung et al (2011) and refined by Manh et al (2014). It is a quasi-2-D model, and takes the complex hydraulic system of the VMD into account.…”

“…However, Howie (2005), Käkönen (2008) questioned this claim, because impeding floodplain inundation reduces the input of sediments and thus of natural fertilizers to the paddy fields. This leads to reduced crop yields (Manh et al, 2014) and, consequently, intensified use of agrochemicals and higher costs. Howie (2005) and Käkönen (2008) also pointed to other social and environmental adverse consequences, e.g.…”

“…To obtain a better understanding of the trends, changes in flood peak and flood duration are analysed, which in combination define the severity of floods in the Mekong Delta . To quantify the relative contribution of high-dyke development, we update the quasi-2-D flood model developed by Dung et al (2011) and Manh et al (2014) using the latest comprehensive dyke survey data and high-resolution topographical data from the newly available 5 × 5 m resolution lidar DEM for the VMD. Changes in delta inundation dynamics for the preand post-high-dyke development are investigated in different model setups, simulating the two recent, most severe flood events in 2000 and 2011.…”

Simulating Future Flows and Salinity Intrusion Using Combined One- and Two-Dimensional Hydrodynamic Modelling—The Case of Hau River, Vietnamese Mekong Delta

“…The rice fields and the floodplain are considered to be artificial channels or "virtual channels", having wide cross sections and were extracted from the digital elevation model (DEM). The calibration process of these channels and crest of dikes was explained in detail in [43,44].…”